Usually, the prior art uninterruptible power system supplies AC power from a power line, such as the utility line 1 to a load 4 such as a computer, communication equipment, or the like through a bypass line 10, as shown in FIG. 5 . When the utility power fails, DC power from a storage battery 6' is converted into AC power by an inverter 3 and supplied to the load 4. In this way, the load 4 is prevented from causing a trouble in the event of a power failure.
The storage battery 6' of this uninterruptible power system is constantly electrically fully charged with DC power which is obtained by rectifying the AC power from the power line 1 by a rectifier 2. This ensures back-up in the event of an unexpected power failure.
In recent years, such uninterruptible power systems have been required to satisfy various needs such as maintenance free miniaturization, and lower costs. Accordingly, there is a strong tendency that the storage battery 6' is enclosed with higher hermeticity. Also, there is an increasing tendency to mount the components in the system at a higher density. In particular, a sealed lead-acid storage battery 6 has been employed as the storage battery 6'. Components including this lead-acid storage battery 6 have been mounted at a high density within the enclosure.
A positive plate used in the sealed lead-acid battery 6 comprises a grid loaded with lead dioxide that is an active material. The grid is made of a lead alloy. As the battery is electrically charged, the surface of the grid is oxidized and corroded, so that the grid increases in volume. This produces internal stress or deteriorates the strength of the grid.
Therefore, where the uninterruptible power system is used under a floating charge condition, the grid grows as the floating charge continues. As a result, a short-circuit to the negative strap, breakage of the battery container, and leakage of the liquid tend to occur.
Generally, as the charged ampere-hour increases, the growth of the grid increases. For the sealed lead-acid battery 6 in which components are mounted at such a high density, the temperature of the battery tends to rise in particular. Therefore, the charged ampere-hour increases, and the temperature of the battery further rises.
Some systems are equipped with a temperature compensation circuit to lower the floating charge voltage with increasing temperature. The practical procedure is to replace the sealed lead-acid battery 6 every two or three years to maintain the reliability.